In recent years, there have been proposed and developed various variable valve timing control systems each employing a phase converter, such as a hydraulically-operated vane-type timing variator, a hydraulically-operated helical-gear-type timing variator, and the like. A hydraulically-operated vane-type timing variator has been disclosed in Japanese Patent Provisional Publication No. 2001-271616 (hereinafter is referred to as “JP2001-271616”), corresponding to German patent application No. 101 01 938 and also corresponding to U.S. Pat. No. 6,345,595, issued on Feb. 12, 2002 and assigned to the assignee of the present invention. In the hydraulically-operated vane-type variable valve timing control system disclosed in JP2001-271616, a vane member is fixedly connected to a camshaft end and rotatably enclosed in a cylindrical housing of a timing pulley whose opening ends are enclosed with front and rear covers. A phase-advance hydraulic chamber and a phase-retard hydraulic chamber are defined between diametrically-opposing partition walls and two blades of the vane member. The hydraulically-operated phase converter operates to vary a relative angular phase between the camshaft and the timing pulley (engine crankshaft) by supplying hydraulic pressure discharged from a reversible pump selectively to either one of the phase-advance hydraulic chamber and the phase-retard hydraulic chamber by switching one of a normal-rotational direction and a reverse-rotational direction of the reversible pump to the other, for variably adjusting a valve open timing and/or a valve closure timing of the engine valve depending on an engine operating condition.